1. Field of Invention
This invention pertains to an amplifier capable of different operating modes and different power supply voltages, and particular to an amplifier for a brushless DC motor.
2. Related Art and Other Considerations
Motor amplifiers are generally designed to operate in only one of a plurality of different modes. Some amplifiers operate in a linear mode; other amplifiers operate in a switching mode. Switching mode amplifiers can further be classified as either two quadrant switching or four quadrant switching. As is well known in the art, each mode is useful under certain circumstances, but has drawbacks that make use thereof less desirable in other circumstances.
Consider a motor driven by bridge transistors, such as a brushless three-phase DC motor, for example. Motor rotation is achieved by commutation, i.e., selectively turning on and off coils in the motor using transistors in the bridge.
Linear mode operation generates the least electronic noise, owing to the absence of switching except at relatively infrequent commutation boundaries. The only switching action occurs when an active coil pair needs to be change to a different pair as the motor rotates. The disadvantage with linear mode operation is that power dissipation can be quite high, resulting in undesirable heating.
Switching mode operation involves switching the bridge transistors on and off at an extremely fast rate (on the order of one hundred thousand times per second). This so-called "chopping" action occurs much faster (typically one hundred times faster) than the commutation activity. The advantage of the switching mode is much lower power dissipation in the amplifier. The disadvantage is increased electronic noise generated by the rapid switching activity.
A certain method of selecting which bridge transistors are turned on and off is known as two quadrant switching mode. In order to make current build up in two of the motor's coils, one upper and one lower transistor are turned on. Due to the inductive nature of the coils, current builds up at a relative slow rate. Typically before the current has reached its maximum (i.e., saturation) value, one of the two transistors is turned off. If the motor speed is not excessive, the current will then "flyback" through one of the diodes associated with the transistor that was just turned off, and flow in a circular path as it slowly decays. By adjusting the ratio of "on" (two transistors on) and "off" (only one transistor on), an average level of motor current can be achieved. The percentage "on" time (on/(on+off)) is called the duty cycle. The small amount (compared with its average value) that the current increases and decreases is called "ripple".
Two quadrant switching works well in cases where motor speed is low and the desired direction of current matches the motor direction. It has the advantage of a relatively slow decay rate of the current during the "off" time, resulting in low current ripple (i.e., the current is relatively constant). However, if rapid deceleration of the motor from a high speed is required, the current in a two quadrant system may fail to decay during the "off" cycle, and in fact increase due to the effect of the motor's internally generated voltage. This results in a complete loss of control.